Isolation of Lolicine A, Lolicine B, Lolitriol, and Lolitrem N from Lolium

Dylan B. England, Jakob Magolan, and Michael A. Kerr. Organic ... Wayne A. Gatenby, Sarah C. Munday-Finch, Alistair L. Wilkins, and Christopher O. Mil...
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J. Agric. Food Chem. 1998, 46, 590−598

Isolation of Lolicine A, Lolicine B, Lolitriol, and Lolitrem N from Lolium perenne Infected with Neotyphodium lolii and Evidence for the Natural Occurrence of 31-Epilolitrem N and 31-Epilolitrem F Sarah C. Munday-Finch,† Alistair L. Wilkins,‡ and Christopher O. Miles*,† New Zealand Pastoral Agriculture Research Institute Ltd., Ruakura Agricultural Research Centre, Private Bag 3123, Hamilton, New Zealand, and Chemistry Department, The University of Waikato, Private Bag 3105, Hamilton, New Zealand

Lolicines A (1a) and B (2a), late-eluting lolitrem-like compounds, were identified in extracts of perennial ryegrass (Lolium perenne) seed that was infected with the endophytic fungus Neotyphodium lolii. The lolicines were isolated as their 11-O-propionates (1b and 2b) and their structures determined by mass spectrometry and one- and two-dimensional NMR spectroscopy. The structures of lolicines A and B were similar to those of paspaline (5a) and paspaline B (5b), compounds known to be biosynthetic precursors of several groups of more complex indolediterpenoids, suggesting that the lolicines might be biosynthetic precursors of the lolitrem group of indole-diterpenoid neurotoxins. During purification of 1b and 2b, a third compound, lolitrem N (4c), was isolated as its propionate (4d) and identified as 35-epilolitriol. A further late-eluting compound was isolated as its acetate from the same fraction as the lolicines. This compound was found to be identical with lolitriol 10-O-acetate (4b) prepared from lolitrem B (3a). This finding confirms lolitriol (4a) as a natural constituent of endophyte-infected perennial ryegrass, and is consistent with the proposal that lolitriol is a biosynthetic precursor of lolitrems A (9), B (3a), and E (10). Detailed examination of the 1H NMR spectra of 4b revealed the presence of 31-epilolitrem N 10-O-acetate (4f) as a naturally occurring contaminant, indicating that 31-epilolitrem N (4e) is a metabolite of endophyte-infected ryegrass. Analysis of the NMR spectra of 3a led to a reassignment of the C-19-C-21 resonances of the lolitrems, and identified 31-epilolitrem F (3c) as a natural constituent of L. perenne. Keywords: Acremonium lolii; Neotyphodium lolii; Lolium perenne; endophyte; lolitrem; lolicine; paspaline; lolitriol; tremorgen; ryegrass staggers; neurotoxin; biosynthesis; mycotoxin INTRODUCTION

Many of the world’s grasses, including the agriculturally important perennial ryegrass (Lolium perenne L.) and tall fescue (Festuca arundinacea Schreb.), are infected with Neotyphodium-like endophytic fungi (White et al., 1987). The presence of endophytic fungi has long been suspected of involvement in the toxicity of some of these grasses, including L. perenne and F. arundinacea (Neill, 1941), Lolium temulentum L. (Freeman, 1902), Echinopogon ovatus (G. Forst.) P. Beauv. (Cleland, 1912), and Melica decumbens Thunb. (Marloth, 1913), to animals. Although grazing experiments indicated that the toxicity of Festuca hieronymi Hackel is due to an endophytic fungus as long ago as 1909 [Rivas and Zanolli, cited (with partial English translation) by Cleland (1912)], and Henry and Massey noted in 1911 the similarity between the symptoms of sheep intoxicated on E. ovatus [reported to be endophyte-infected by Cleland (1912)] in Australia and those intoxicated on ryegrass in New Zealand, the role of endophyte in the toxicity of F. arundinacea and L. perenne was not demonstrated until ca. 1980 (Hoveland et al., 1980; Fletcher and Harvey, 1981). Neotyphodium-infected * Author to whom correspondence should be addressed (email [email protected]). † Ruakura Agricultural Research Centre. ‡ The University of Waikato.

grasses indigenous to every continent except Antarctica have now been implicated in the poisoning of grazing animals (Munday-Finch et al., 1997). Perennial ryegrass staggers (RGS) is a neurotoxic disorder of livestock grazing L. perenne infected with the endophytic fungus Neotyphodium lolii (G. MorganJones & W. Gams) A. Glenn, C. W. Bacon & R. T. Hanlin (formerly, Acremonium lolii). The tremors which characterize RGS are thought to be due to ingestion of lolitrems (Gallagher et al., 1981, 1982), tremorgenic indole-diterpenoids produced by the endophyte (di Menna et al., 1991; Miles et al., 1992; Penn et al., 1993). Because of the advantageous characteristics which endophyte infection confers upon L. perenne in New Zealand (Prestidge et al., 1982; Prestidge and Ball, 1993), removal of the endophyte is not at present a viable option. One strategy for prevention of RGS is deliberate infection of L. perenne with endophyte strains selected for their inability to produce tremorgens (Fletcher, 1993; Ralston, 1993). Such an approach would prevent RGS without loss of the beneficial effects of endophyte infection, but requires some knowledge of tremorgen biosynthesis for success to be assured (Penn et al., 1993). The structure of lolitrem B (3a), the principal lolitrem, was elucidated by Gallagher et al. (1984) and its stereochemistry determined by Ede et al. (1994). Recent studies (Miles et al., 1992, 1994, 1995b; Munday-Finch

S0021-8561(97)00678-X CCC: $15.00 © 1998 American Chemical Society Published on Web 01/22/1998

Lolitrem Analogues from Endophyte-Infected Ryegrass

et al., 1995, 1996b, 1997) have revealed the structures of several of the less abundant lolitrems, allowing inferences to be made about lolitrem biosynthesis and structure-activity relationships (Miles et al., 1992, 1994, 1995c,d; Munday-Finch et al., 1995, 1996b, 1997). As a continuation of this work, we report here on an investigation of polar lolitrems from extracts obtained during earlier isolations of lolitrem B (Miles et al., 1994), and on a detailed NMR examination of 3a. Preliminary reports of parts of this work have been published (Miles et al., 1995b,c; Munday-Finch, 1997). EXPERIMENTAL PROCEDURES General. Mass spectra were obtained with a direct insertion probe on a Kratos MS-80 RFA instrument. Flash chromatography (Still et al., 1978) was performed with silica gel (Merck, art. 9385). The lolitrem content of fractions obtained during purification was assessed by HPLC, based on the method of Gallagher et al. (1985) as modified by Miles et al. (1994), with a 4.6 mm × 25 cm Zorbax silica gel column (5 µm) and acetonitrile-dichloromethane (1:4 or 7:13, as appropriate) as the eluent (1.8 mL min-1). Eluting compounds were detected with a Shimadzu RF-530 Fluorescence Spectromonitor (excitation at 268 nm, emission detection at 440 nm) and a Hewlett-Packard 1040M diode array UV detector connected in series. Semipreparative HPLC purification was performed on an RCM-100 Radial Compression Separation System (Waters) fitted with a silica gel Radial-PAK cartridge (8 mm × 10 cm, 10 mm) (Waters), with acetonitrile-dichloromethane (1:4 or 1:3, as appropriate) as the eluent (3.0 mL min-1). Eluting compounds were detected with an LC-85B spectrophotometric detector (Perkin-Elmer). Nuclear Magnetic Resonance Spectroscopy. All compounds were dissolved in deuteriochloroform (CDCl3) for NMR spectroscopy, and chemical shifts, determined at 300 K, are reported relative to TMS based on internal standards (δ1H CHCl3 ) 7.26, δ13C CDCl3 ) 77.1). One-dimensional 1H (300.13 MHz) and 13C (75.47 MHz) NMR spectra of 2b and 4b were obtained with a Bruker AC-300 instrument. Heteronuclear multiple-bond coherence (HMBC), heteronuclear multiple-quantum coherence (HMQC), and heteronuclear singlequantum coherence (HSQC) spectra were acquired with pulsedfield gradients (Braun et al., 1996); correlated spectroscopy (COSY), rotating-frame Overhauser enhancement spectroscopy (ROESY) (phase sensitive, with the excitation frequency offset by at least 0.5 ppm from any signal of interest), and 1H NMR spectra of 1b, 2b, 3a, 4b, and 4d were obtained on a Bruker DRX-400 MHz instrument. Isolation of Lolitriol (4a). A fraction enriched in latereluting lolitrem compounds (by HPLC analysis) was obtained during the isolation of lolitrem B (Miles et al., 1994) from 360 kg of L. perenne seed. Initial purification of this fraction was by flash column chromatography with a stepwise gradient of acetonitrile-dichloromethane (1:19, 1:9, 3:17, 1:4, 3:7, 2:3, 1:1, and 7:3) as the eluent. Further flash chromatography, with a stepwise gradient of acetonitrile-dichloromethane (1:19, 1:9, 3:17, 1:4, 1:3, 3:7, 7:13, 2:3, 3:2, and 7:3) as the eluent, afforded two fractions, one of which contained predominantly lolitriol and the other predominantly lolicines. The lolitriol fraction was further purified by flash column chromatography with acetonitrile-dichloromethane (3:22 or 3:17) and then again with ethyl acetate-petroleum spirit (3: 17) (each containing 1% v/v methanol) as eluents. The resulting crude lolitriol was dissolved in the minimum volume of dichloromethane, to which an equal volume of acetonitrile was then added, and the solution was slowly concentrated on a rotary evaporator to precipitate the majority of the contaminants. The supernatant, which contained the lolitriol, was then purified by semipreparative HPLC. Final purification was achieved by treatment with acetic anhydride-pyridine (1: 1, 6 mL) for 24 h at 4 °C followed by flash column chromatography with acetonitrile-dichloromethane (1:9) as the eluent to yield lolitriol 10-O-acetate (4b) as a colorless solid (3 mg).

J. Agric. Food Chem., Vol. 46, No. 2, 1998 591 1H NMR: δ 2.78 (d, J ) 14.1 Hz, H-31), 3.45 (dd, J ) 15.8, 3.6 Hz, H-36β), 3.53 (d, J ) 9.4 Hz, H-9R), 3.64 (s, H-11β), 4.24 (br t, J ) 8.9 Hz, H-7R), 5.13 (dd, J ) 9.3, 1.3 Hz, H-10β), 7.23 (d, J ) 8.7 Hz, H-23), 7.87 (d, J ) 8.7 Hz, H-22). Full 1H and 13 C NMR assignments for lolitriol 10-O-acetate are reported in Table 1. EI-MS m/z (relative intensity): 662 (45), 661.3623 (M+, 661.3615 for C39H51NO8, 100), 647 (34), 349 (6), 348 (17). Preparation of Lolitriol 10-O-Acetate (4b) from Lolitrem B (3a). A specimen of lolitrem B was subjected to acidic hydrolysis (Miles et al., 1992), and the resulting lolitriol (4a) was acetylated as above. The product was purified by flash column chromatography with acetonitrile-dichloromethane (1:9) as the eluent to give lolitriol 10-O-acetate (4b). This material was identical in all respects to that isolated from L. perenne extracts (above). Isolation of Lolicine A, Lolicine B, and Lolitrem N. A fraction containing lolicines was obtained during the purification of lolitriol (above). This was purified by flash column chromatography with acetonitrile-dichloromethane (7:43, and 1% v/v methanol) and then again with a stepwise gradient of ethyl acetate-petroleum spirit (3:17, 1:4, 3:7, and 1:1, each with 1% v/v methanol added) as the eluents. Treatment of the resulting oil with propionic anhydride-pyridine (1:1, 2 mL) for 36 h at room temperature, followed by flash column chromatography with acetonitrile-dichloromethane (1:9, and 1% v/v methanol) as the eluent, yielded two fractions; one contained mainly lolicine A propionate (1b) and the other mainly lolicine B propionate (2b). Lolicine A 11-O-propionate was purified by flash column chromatography with a stepwise gradient of acetonitrile-dichloromethane (7:43, 17:83, and 3:7) as the eluent, followed by semipreparative HPLC to give lolicine A 11-O-propionate (1b) as a colorless solid (0.2 mg). 1H NMR: δ 2.52 (dd, J ) 12.9, 10.3 Hz, H-17R), 2.77 (d, J ) 14.1 Hz, H-31), 2.90 (dd, J ) 12.9, 6.3 Hz, H-17β), 2.96 (dd, J ) 15.5, 12.4 Hz, H-36R), 3.43 (dd, J ) 15.9, 3.8 Hz, H-36β), 3.45 (dd, J ) 12.4, 3.0 Hz, H-9R), 4.96 (t, J ) 2.9 Hz, H-11β), 7.21 (d, J ) 8.7 Hz, H-23), 7.87 (d, J ) 8.7 Hz, H-22). 1H and 13C NMR data for 1b are presented in Table 1. EI-MS m/z (relative intensity): 660 (52), 659.4192 (M+, 659.4188 for C41H57NO6, 100), 645 (21), 644 (49), 567 (14), 552 (14), 355 (25), 349 (18), 348 (52), 296 (12), 143 (28). Further purification of the fraction containing lolicine B propionate was achieved by flash chromatography with acetonitrile-dichloromethane (1:4) as the eluent. Purification by semipreparative HPLC separated this fraction into lolicine B 11-O-propionate (2b) (0.3 mg) and lolitrem N 10-O-propionate (4d) (0.3 mg), obtained as colorless solids. Lolicine B 11-Opropionate (2b) 1H NMR: δ 2.51 (dd, J ) 13.3, 10.5 Hz, H-17R), 2.76 (d, J ) 14.1 Hz, H-31), 2.91 (dd, J ) 13.5, 6.5 Hz, H-17β), 2.95 (dd, J ) 16.7, 11.7 Hz, H-36R), 3.42 (dd, J ) 16.1, 4.0 Hz, H-36β), 3.48 (dd, J ) 12.0, 2.5 Hz, H-9R), 3.82 (dd, J ) 12.2, 4.7 Hz, H-7R), 5.45 (t, J ) 3.0 Hz, H-11β), 7.20 (d, J ) 8.7 Hz, H-23), 7.87 (d, J ) 8.7 Hz, H-22). 1H and 13C NMR data for 2b are presented in Table 1. Lolicine B 11-O-propionate (2b) EI-MS m/z (relative intensity): 674 (56), 673.3977 (M+, 673.3981 for C41H55NO7, 100), 658 (51), 640 (13), 571 (13), 556 (21), 529 (12), 349 (15), 348 (60), 296 (15), 238 (11), 205 (62). Lolitrem N 10-O-propionate (4d) 1H NMR: δ 3.20 (dd, J ) 16.8, 11.9 Hz, H-36β), 3.29 (dd, J ) 16.8, 6.8 Hz, H-36R), 3.35 (d, J ) 7.3 Hz, H-31). 3.53 (d, J ) 9.4 Hz, H-9R), 3.63 (s, H-11β), 4.24 (t, J ) 9.2 Hz, H-7R), 5.13 (dd, J ) 9.4, 1.0 Hz, H-10β), 7.23 (d, J ) 8.6 Hz, H-23), 7.85 (d, J ) 8.6 Hz, H-22). 1H and 13C NMR data for 4d are presented in Table 1. Lolitrem N 10-O-propionate (4d) EI-MS m/z (relative intensity): 676 (42), 675.3740 (M+, 675.3771 for C40H53NO8, 94), 661 (42), 660 (100), 657 (53), 642 (52), 471 (10), 456 (12), 349 (29), 348 (64), 335 (12), 290 (22), 238 (17), 149 (33).

RESULTS AND DISCUSSION

Structure Elucidation of Lolicine A (1) and Lolicine B (2). The UV spectra of 1b and 2b were very similar to that of 3a (Figure 2), suggesting the presence of a similar chromophore (rings B-D) in these com-

592 J. Agric. Food Chem., Vol. 46, No. 2, 1998

Munday-Finch et al.

Table 1. 1H and 13C NMR Assignments (δ, CDCl3) for Lolitrem B (3a), Lolicine A 11-O-Propionate (1b), Lolicine B 11-O-Propionate (2b), Lolitriol 10-O-Acetate (4b), Lolitrem F (3b), and Lolitrem N 10-O-Propionate (4d) lolitrem B (3a)a atom 2 3 4 5 6 7 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 34 35 36 37 38 39 40 41 43 44 45 46 47 CO2 CH2 CH3

13C

152.8 50.7 42.4 27.4 28.0 71.5 71.2 71.1 61.3 67.7 78.1 30.3 20.5 50.1 29.2 118.6 123.9 137.0 125.4 120.2 110.4 142.0 15.9 18.9 74.7 28.3 16.6 196.5 59.9 79.9 79.3 49.9 28.3 30.6 25.1 25.0 29.3 92.7 122.0 139.5 18.6 25.6

1Hb

2.70, 1.36 2.27, 1.76 4.33 3.57 3.92 3.63 1.56, 1.42 1.95, 1.64 2.86 2.63, 2.94

7.87 7.22 1.28 1.15 1.30 1.30 2.78 2.68 2.98, 3.44 1.54 1.32 1.26 1.39

lolicine A 11-Opropionate (1b) 13C

151.5 53.2 39.9 33.5 24.5 78.5 79.7 27.0 72.3 40.5 39.0 25.0 21.7 49.5 29.5 nd nd nd nd 120.6 110.5 nd 14.5 20.0 71.4 26.3 23.7 nd 60.0 79.9 79.1 49.9 28.4 30.7 25.1 25.1 29.5 12.8

1Hb

1.96, 1.61 1.86, 1.79 3.61 3.45 2.00, 1.57 4.96 1.95 1.65, 1.45 1.45, 1.79 2.79 2.52, 2.90

7.87 7.21 1.03 1.17 1.18 1.14 2.77 2.68 2.96, 3.43 1.53 1.32 1.25 1.38 0.96

lolicine B 11-Opropionate (2b) 13C

151.1 52.5 39.5 32.9 24.4 77.8 79.8 29.1 69.0 53.9 42.5 22.1 24.6 48.2 29.7 119.0 124.0 nd nd 120.9 110.5 142.1 14.8 19.9 71.4 25.8 23.8 196.8 60.3 80.0 79.3 49.7 28.3 30.7 25.1 25.1 29.4 204.6

1Hb

2.03, 1.73 2.10, 2.30 3.82 3.48 1.75, 1.50 5.45 2.05 1.65, 1.50 1.48, 1.80 2.70 2.51, 2.91

7.87 7.20 1.01 0.96 1.15 1.10 2.76 2.69 2.95, 3.42 1.53 1.31 1.24 1.38 10.15

lolitriol 10-Oacetate (4b) 13C

152.6 50.7 42.2 27.3 27.5 71.6 75.7 68.1 62.3 70.2 77.7 30.4 20.5 50.0 29.2 118.8 124.0 137.1 126.1 120.4 110.5 142.0 16.0 19.0 71.2 27.0 24.7 196.6 60.0 80.0 79.3 49.9 28.3 30.7 25.1 25.1 29.4

1Hb

2.75, 1.37 2.37, 1.85 4.24 3.53 5.13 3.64 1.57, 1.45 1.96, 1.68 2.85 2.64. 2.95

7.87 7.23 1.28 1.16 1.21 1.15 2.78 2.68 2.96, 3.45 1.54 1.32 1.26 1.39

5.54 5.30

lolitrem F (3b)a 13C

152.6 50.7 42.4 27.6 28.0 71.5 71.3 71.1 61.2 67.8 78.1 30.3 20.6 50.7 29.0 118.9 123.6 136.5 125.3 120.2 110.6 142.0 16.1 19.0 74.8 28.3 16.7 197.1 57.6 82.7 82.1 47.9 25.7 33.5 27.8 29.2 24.4 92.7 122.0 139.7 18.7 25.7

1.73 1.75 173.6 28.1 9.3

2.42 1.23

172.6 28.0 9.0

Ester Signals 170.3 2.43 1.24 21.3

2.13

1Hb

2.73, 1.36 2.29, 1.75 4.33 3.57 3.92 3.62 1.56, 1.42 1.99, 1.64 2.85 2.63, 2.90

7.84 7.22 1.30 1.15 1.30 1.30 3.35 2.67 3.30, 3.19 1.59 1.11 1.36 1.43

lolitrem N 10-Opropionate (4d) 13C

152.3 50.5 42.3 27.2 27.5 71.6 75.5 68.1 62.2 ndc 77.7 30.5 20.5 51.2 29.0 nd nd 136.5 nd 120.3 110.5 nd 16.0 18.9 70.9 27.0 24.6 197.3 57.5 82.6 82.0 47.9 25.5 33.3 27.6 29.1 24.4

1Hb

2.75, 1.38 2.38, 1.85 4.24 3.53 5.13 3.63 1.42-1.57 1.94, 1.64 2.81 2.62, 2.89

7.85 7.23 1.30 1.16 1.20 1.15 3.35 2.68 3.29, 3.20 1.59 1.11 1.36 1.43

5.54 5.30 1.73 1.74 173.3 27.7 9.5

2.42 1.17

a From the data of Munday-Finch et al. (1996b), with revised assignments for C-19-C-21. b Methylene proton resonances in the format HR, Hβ. c nd ) not detected.

pounds. This indicated that 1a and 2a, and their propionate esters, probably possessed trans-fused ring A/B systems as in 3a rather than the cis-fused ring A/B system of lolitrem F (3b) (Munday-Finch et al., 1996b). The lolicines eluted much later on HPLC (Figure 2) than 3a, indicating that they are more polar than 3a. Furthermore, reaction of 1a and 2a with propionic anhydride yielded monopropionates, suggesting that one primary or secondary hydroxyl group was present in their structures. Mass spectral analysis of the propionate of 1a (i.e., 1b) showed a molecular ion at m/z 659.4192, consistent with C41H57NO6, indicating that 1a is C38H53NO5. The characteristic ion at m/z 348 was present in the mass spectrum of 1b (see Figure 1), consistent with the presence of the ring A-E structure that is present in 3a (Gallagher et al., 1984). These results suggested that the A-E ring system present in 3a was also present in 1b, with the differences being in rings F-H.

The 1H NMR spectrum of 1b exhibited nine tertiary methyl group signals, one more than that of 4a. A single C(H)OCOEt signal (H-11, 4.96 ppm) was also observed. The 1H NMR spectrum of 1b was also characterized by a lack of signals that could be attributed to an acetal moiety like that present in 3a (H43-H-47). In addition, the triplet (H-7) and singlet (H9) present in the 1H NMR spectrum of 3a were instead present as complex multiplets in the 1H NMR spectrum of 1b, suggesting substantial differences between 3a and 1b in ring H. The two-dimensional COSY spectrum of 1b demonstrated that the C(H)OCOEt signal (4.96 ppm) coupled to two mutually coupled protons (2.00 and 1.57 ppm), each of which coupled to H-9 (3.45 ppm). Thus, the protons at 2.00 and 1.57 ppm were identified as H-10 methylene protons, with the propionate attached to

Lolitrem Analogues from Endophyte-Infected Ryegrass

J. Agric. Food Chem., Vol. 46, No. 2, 1998 593

Figure 1. Structures of lolicine A (1a) and its 11-O-propionate (1b), lolicine B (2a) and its 11-O-propionate (2b), lolitrem B (3a), lolitrem F (3b), 31-epilolitrem F (3c), 31-epilolitrem B (3d), lolitriol (4a) and its 10-O-acetate (4b), lolitrem N (4c) and its 10-Opropionate (4d), 31-epilolitrem N (4e) and its acetate (4f), paspaline (5a), paspaline B (5b), and terpendoles E (6a), F (6b), and G (6c).

C-11. The acetal moiety (C-43-C-47) and epoxide group (C-11-C-12) of 3a are therefore absent in 1b. Thus, eight (C-25, C-26, C-28, C-29, and C-37-C-40) of the nine tertiary methyl resonances of 1b were accounted for. Due to the small quantity of 1b available, it was not possible to obtain a 13C NMR spectrum. However, an inverse-mode two-dimensional HMBC experiment (Supporting Information) allowed the assignment of many of the carbon resonances of 1b (Table 1). Of particular importance was the observation that H-26 (1.17 ppm) did not correlate with any oxygenated carbons, indicating that a 13-OH group was not present in 1b. The ninth methyl group (0.96 ppm) correlated to two oxygenated carbons (at 72.3 and 78.5 ppm). The oxygenated carbons in rings F-H are C-7, C-9, C-11, and C-27, and the only positions close to two such carbons are C-9, C-10, and C-12. Because H-9 (3.45 ppm) and H-10 (2.00 and 1.57 ppm) were identified in the two-dimensional COSY experiment (above), the ninth methyl group (C41) was therefore attached to C-12. A ROESY correlation between H-26 (1.17 ppm) and the methyl singlet at 0.96 ppm (H-41) located C-41 on the β-face. A ROESY correlation was also observed between H-11 (4.96 ppm) and H-41, indicating that H-11 was also β-oriented. The above observations indicate that the structure of lolicine A 11-O-propionate (1b) is as shown in Figure 1, and that lolicine A has structure 1a. Mass spectral analysis of 2b showed a molecular ion at m/z 673.3977, consistent with the formula C41H55NO7 (and thus with C38H51NO6 for 2a). Again, the characteristic ion at m/z 348 was present, indicating

that rings A-E of 3a were present in 2b, with the differences being in rings F-H. The 1H NMR spectrum of 2b showed eight (rather than nine, as in 1b) tertiary methyl group signals. In addition, the C(H)OCOEt signal (5.45 ppm) of 2b was shifted downfield by 0.49 ppm relative to that of 1b. A feature of the 1H NMR spectrum of 2b was the presence of a signal at 10.15 ppm, indicative of an aldehyde proton. The replacement of a methyl group with a formyl group would account for the difference of 14 mass units between 1b and 2b, and for there being one less tertiary methyl group in 2b than in 1b. The two-dimensional COSY and HMBC spectra of 2b showed correlations very similar to those observed for 1b. The H-10 methylene protons were again detected in the COSY spectrum of 2b, indicating the absence of the acetal moiety that is present in 3a. The HMBC spectrum (Supporting Information) showed that the C-12 methyl of 1b was absent in 2b, establishing that the formyl group was attached to C-12 of 2b. The formyl group was shown to be β-oriented by the observation of a ROESY correlation between the formyl proton (H-41, 10.15 ppm) and H-26 (0.96 ppm). The structure of lolicine B 11-O-propionate was thus established as 2b (Figure 1), and lolicine B therefore has structure 2a. The structures established for lolicines A (1a) and B (2a) are directly analogous to those of terpendoles E (6a) and G (6c) (Tomoda et al., 1995), respectively, with the latter compounds differing only by the absence of the lolitrem A/B ring system that is present in the lolicines (Figure 1). Structure Elucidation of Lolitriol. The mass spectrum of 4b contained an ion at m/z 661.3623, consistent with a molecular ion of the formula C39H51-

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Figure 2. (A) HPLC chromatogram, with acetonitriledichloromethane (7:13) as eluent and fluorescence detection, of the fraction containing lolicines A (1a) and B (2a), lolitriol (4a), and lolitrem N (4c). Insets show normalized UV absorbance spectra of (B) lolicine A 11-O-propionate (1b), lolicine B 11-O-propionate (2b), lolitrem B (3a), and lolitriol 10-O-acetate (4b) and (C) lolitriol 10-O-acetate (4b) and lolitrem N 10-Opropionate (4d). The spectra were obtained from HPLC chromatograms, with acetonitrile-dichloromethane (1:4) as eluent, by means of a diode array detector.

NO8. The mass spectrum also contained a prominent ion at m/z 348, consistent with the presence in 4b of the A-E ring system that is present in 3a. The similarity of the UV spectrum of 4b to that of 3a (Figure 2) was also consistent with the presence of a trans-fused ring A-D system like that in 3a, rather than the cisfused system that is present in lolitrem F (3b) (MundayFinch et al., 1996b). The 1H NMR spectrum revealed that 4b was a monoacetate, and that 4a therefore had the formula C37H49NO7. The 1H and 13C NMR spectra of 4b were found to be identical with those of authentic lolitriol 10-O-acetate prepared from lolitriol (4a) (Miles et al., 1992) by acetylation. The assignments of the NMR spectra of 4b (Table 1) were determined in a manner analogous to that used in the NMR assignments of 1b and 2b (above). For example, the COSY spectrum of 4b included correlations which readily identified the H-7, H-9, H-10, and H-11 resonances. Compound 4b was thus identified as lolitriol 10-O-acetate (Figure 1). The absence of 4b (by HPLC) in the lolicine-containing fraction prior to acetylation indicates that lolitriol (4a) was present in the L. perenne extract. This is in accord with the earlier identification of lolitriol by HPLC in endophyte-infected L. perenne (Miles et al., 1992) and in cultures of N. lolii (di Menna et al., 1991; Miles et al., 1992; Penn et al., 1993). Close inspection of the 1H and COSY NMR spectra of 4b (see the Supporting Information; note that δ 2.553.55 of these spectra were very similar in appearance to those for 3a shown in Figure 3) revealed the presence of multiplets that could be attributed to the presence of ca. 15% of the 31β,35R-epimer of lolitriol 10-O-acetate (4f). Since 4f is the 31β-epimer of lolitrem N 10-Oacetate (below), the designation 31-epilolitrem N 10-Oacetate is proposed. The H-36R signal (3.34 ppm, dd, J

Munday-Finch et al.

Figure 3. COSY NMR spectrum of lolitrem B (3a), from δ 2.55-3.55, showing correlations arising from H-36R of contaminating 31-epilolitrem F (3c).

) 3.9, 15.8 Hz) of 4f was completely resolved from the H-36β signal of 4b. The H-36R signal of 4f exhibited COSY correlations to signals centered at 3.01 (H-36β) and 2.70 (H-35R). The chemical shifts of H-36R (3.34 ppm) and H-36β (3.01 ppm) of contaminant 4f corresponded closely with those previously observed by Munday-Finch et al. (1996b) (3.34 and 2.99 ppm, respectively) for the 31β,35R-epimer of lolitrem B (3c). Structure Elucidation of Lolitrem N (4c). The UV spectrum of lolitrem N propionate (4d) was similar to those of other lolitrems (Figure 2), suggesting the presence of the lolitrem (rings B-D) chromophore. However, its absorbance maximum was shifted by 2 nm toward longer wavelengths relative to that of lolitriol 10-O-acetate (4b). A similar shift has been observed in the UV absorption maxima of lolitrems containing cis-fused (e.g., 3b and 3d), rather than the more common trans-fused (e.g., 3a and 3c), A/B ring junctions (Figure 1) (Munday-Finch et al., 1996b), suggesting that lolitrem N contained a cis-fused A/B ring junction. Lolitrem N (4c) appeared to coelute with lolicine B (2a) during analytical HPLC. When reacted with propionic anhydride, it gave a compound (4d) that coeluted with lolicine B 11-O-propionate (2b) on analytical HPLC and showed a single set of propionate ester signals in its 1H NMR spectrum, indicating that lolitrem N (4c) contained one primary or secondary hydroxyl group. Mass spectral analysis of propionate 4d showed a molecular ion at m/z 675.3740, consistent with C40H53NO8, indicating that 4c is C37H49NO7. The 1H NMR spectrum of 4d showed the presence of eight singlet methyl groups, two fewer than for 3a. A number of characteristic features observed in the 1H

Lolitrem Analogues from Endophyte-Infected Ryegrass

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Figure 4. Possible metabolic grid for biosynthesis of lolitriol, thought to be the biosynthetic precursor of lolitrem B (Miles et al., 1992), involving paxilline (7), the paspalines (5a and 5b), terpendoles E (6a), G (6c), and I (Tomoda et al., 1995), and the lolicines (1a and 2a).

NMR spectrum of 4b were also present in the spectrum of 4d. These included a lack of signals that could be attributed to the acetal moiety and the presence of a proton signal at 5.13 ppm. The latter is characteristic of a proton located next to an O-acyl group, as in lolitriol 10-O-acetate (4b) (H-10, 5.13 ppm). Furthermore, the chemical shift of H-7 in 4d was the same as that of the corresponding proton of lolitriol acetate (4b). Like that of 3b (Munday-Finch et al., 1996b) but unlike that of 4b, however, H-31 of 4d resonated at 3.35 ppm with a JH-31-H-35 of 7.3 Hz, consistent with a cis A/B ring junction (Munday-Finch et al., 1996b). The 1H NMR therefore suggested that propionate 4d was an analogue of lolitriol 10-O-acetate (4b) containing a modified A/B ring junction. This was consistent with the mass spectral data, which indicated that the atomic compositions of the parent alcohols (i.e., 4a and 4c) of ester derivatives 4b and 4d were the same. The COSY spectrum of 4d allowed most of the proton resonances to be assigned. In particular, cross-peaks were observed between H-9 (3.53 ppm) and both H-11 (3.63 ppm) and the proton resonance at 5.13 ppm (H10). The value for JH-9R-H-10 (9.4 Hz) indicated that H-10 is β-oriented (Miles et al., 1992). The COSY spectrum, optimized for long-range couplings, showed two pairs of 4J-coupled A ring methyl groups, one of which (H-37 and H-38) was at 1.59 and 1.11 ppm, with the other pair (H-39 and H-40) at 1.43 and 1.36 ppm. This pattern is characteristic (Table 1) of a cis-fused A/B ring system such as that in 3b, and not of a trans-fused A/B ring system such as that in 3a (Munday-Finch et al., 1996b). Although insufficient material was available for a 13C NMR spectrum of 4d, the HMBC experiment allowed the assignment of a number of carbon resonances. In addition to the correlations determined for the methyl group protons, a correlation between H-31 and C-30 (197.3 ppm) was also observed (Supporting Information). The HMQC spectrum of 4d allowed assignment of the methine, methylene, and methyl 1H and 13C resonances. These assignments (Table 1) showed that

signals that could be attributed to rings F-H of 4d corresponded very closely to those of lolitriol 10-Oacetate (4b). In contrast, the signals that could be attributed to rings A-C of 4d corresponded very closely to those of lolitrem F (3b) (Table 1). The structure of lolitrem N and its propionate were therefore established as 4c and 4d, respectively (Figure 1). NMR Analysis of Lolitrem B (3a). HMBC correlations were observed from the NH proton, H-22, and H-23 in the 400 MHz, but not in the 300 MHz, NMR spectrum of 3a. These correlations (Supporting Information) indicated that a reassignment of the C-19-C-21 resonances of 3a was necessary (Table 1). The revised assignments for C-21 and C-20 of 3a are in good agreement with those for the analogous carbon resonances (C-2 and C-3) of 2-cyclohexen-1-one (Breitmaier and Voelter, 1987). Similar analyses of HMBC correlations for several other lolitrems (S. C. Munday-Finch, A. L. Wilkins, and C. O. Miles, unpublished observations) indicated that the revised assignments for C-19C-21 were general to all the known lolitrems, and the assignments in Table 1 are presented accordingly. Examination of the 400 MHz 1H NMR spectrum of 3a revealed the presence of a minor contaminant (ca. 5%) which was not apparent in the 300 MHz 1H NMR spectrum. The NMR resonances of the contaminant indicated that it differed from lolitrem B only in the A/B ring system. Examination of COSY correlations (Figure 3) of the contaminant, in a manner analogous to that used to identify the minor contaminant of 4b (above), revealed the presence of a 31β,35R-lolitrem ring A/B system. The contaminant of lolitrem B was thus identified as 31-epilolitrem F (3c). This conclusion is consistent with the previously unexplained observation (Munday-Finch et al., 1996b) that base-catalyzed epimerization of lolitrem B (3a) generated small amounts of lolitrem F (3b) in addition to the expected product [31epilolitrem B (3d)], whereas epimerization of lolitrem F (3b) resulted in formation only of the expected mixture of isomers (3b and 3c). 31-Epilolitrem F (3c)

596 J. Agric. Food Chem., Vol. 46, No. 2, 1998

coelutes with lolitrem B during HPLC analysis (Munday-Finch et al., 1996b), which explains why the presence of small amounts of contaminating 3c has not been previously detected in 3a. Epimerization of lolitrems does not occur under the conditions used for isolation of lolitrem B (3a) (Munday-Finch et al., 1996b), so the 31-epilolitrem F (3c) observed as a contaminant of 3a is therefore a naturally occurring constituent of endophyte-infected L. perenne. A previous attempt to determine the stereochemistry of the ring A/B junction of lolitrem B by observation of the predicted NOE between its H-36 and H-17 protons was hampered by inadequate separation (at 300 MHz) of the H-36R (2.98 ppm) and H-17β (2.94 ppm) resonances. However, an NOE observed in a derivative indicated that lolitrem B possessed structure 3a rather than structure 3c (Ede et al., 1994). The H-36R and H-17β resonances were sufficiently well-resolved in the 400 MHz ROESY NMR spectrum of lolitrem B (3a) (Supporting Information) to enable a medium-intensity correlation between H-36β (3.44 ppm) and H-17β (2.94 ppm) to be identified. The observed correlation is consistent with the structure of lolitrem B (3a) proposed by Ede et al. (1994), whereas molecular modeling indicated that at least two H-17-H-36 correlations would be expected if lolitrem B had the structure of 3c (Supporting Information). Biosynthesis of Lolitrems. Much recent research with regard to reducing the incidence of RGS has concentrated on the development of selected L. perenneN. lolii associations that do not produce significant levels of lolitrem B, but which do produce the insect feeding deterrent peramine (Fletcher, 1993). These grass-endophyte associations should be resistant to Argentine stem weevil (Listronotus bonariensis), a major pest of New Zealand pasture, and yet be unable to cause RGS (Ralston, 1993). In practice, however, introduction of a selected endophyte into a new host plant often alters the expression of toxin biosynthesis by the endophyte (Ralston, 1993). Munday-Finch et al. (1995) suggested that this problem could be avoided by selecting endophytes with defects early in their indole-diterpenoid biosynthetic pathway so that the endophyte would be incapable of producing tremorgens regardless of the host grass into which it was inserted. The success of such an approach depends upon an understanding of tremorgen biosynthesis. Unfortunately, when grown in culture, N. lolii does not produce quantities of lolitrems sufficient for biosynthetic studies (di Menna et al., 1991; Miles et al., 1992; Weedon, 1987); usually, less than 10 µg of lolitrem B is obtained from 100 mL cultures grown for 2 months (M. E. di Menna, J. M. Sprosen, A. D. Hawkes, S. C. Munday-Finch, and C. O. Miles, unpublished observations; Penn et al., 1993). However, the structures of the various lolitrems that have been isolated from endophyte-infected L. perenne provide some clues to the biogenesis of the lolitrems. The confirmation that lolitriol is a natural constituent supports the proposal that lolitriol (4a) is a biosynthetic precursor of lolitrem B (Miles et al., 1992). This work also reveals that 31-epilolitrem F (3c) [a 31,35-epimer of lolitrem B (3a)], as well as lolitrem N (4c) and 31epilolitrem N (4e) [the lolitriol analogues corresponding to lolitrem F (3b) and 31-epilolitrem F (3c), respectively], is a natural constituent of endophyte-infected ryegrass. These findings, and the observation that the

Munday-Finch et al.

relative abundances of 3a, 3b, and 3c parallel those of 4a, 4c, and 4e, are consistent with each lolitriol analogue being a biosynthetic precursor of its corresponding lolitrem B analogue. The biosynthetic machinery for assembling the A/B ring system of the lolitrems appears to lack stereospecificity (MundayFinch et al., 1996b), resulting in a series of lolitrem B analogues which differ only in their stereochemistry at C-31 and C-35. Of the four possible 31,35-stereoisomers of lolitrem B (i.e., 3a-d), the three naturally occurring isomers (3a-c) are all tremorgenic (Munday-Finch et al., 1996b). Nontremorgenic analogue 3d has not yet been detected in L. perenne, and probably does not occur naturally as its presence in ryegrass extracts would be easy to detect because it elutes well clear of other lolitrems upon HPLC analysis (Munday-Finch et al., 1996b).

Lolitriol is thought to be a precursor of lolitrems A (9) (Munday-Finch et al., 1995), B (3a) (Miles et al., 1992), and E (10) (Miles et al., 1994). Previous work has suggested that paxilline (7) (Figure 4), which has been isolated from endophyte-infected L. perenne (Weedon and Mantle, 1987), and R-paxitriol (8) (Miles et al., 1992; Mantle and Weedon, 1994) are precursors in the biosynthesis of lolitriol (4a). Biosynthetic investigations of other indole-diterpenoid-producing fungi indicate that 7, as well as more complex tremorgens such as the penitrems and janthitrems, is produced via the paspalines (5a and 5b) from geranylgeranyl pyrophosphate and tryptophan (Mantle and Weedon, 1994; Munday-Finch et al., 1996a). The lolicines (1a and 2a) contain a methyl or formyl group at C-12, and are therefore unlikely to have been biosynthesized via 7 or 8; rather, their structures suggest that they may be produced by the endophyte via 5a and/or 5b (Figure 4). This contention is strongly supported by the recent identification of paspaline (5a) and 13-desoxypaxilline, as well as a novel terpendole, in endophyte-infected L. perenne seed (Gatenby et al., 1997). Terpendoles E-G (6a-c), close analogues of lolicines A (1a) and B (2a), have recently been isolated from Albophoma yamanashiensis along with an array of lolitrem analogues in which the lolitrem A/B ring system is absent (Huang et al., 1995a,b; Tomoda et al., 1995). The presence in endophyte-infected L. perenne of a terpendole suggests that terpendoles may have a role in lolicine and lolitrem biosynthesis, and the

Lolitrem Analogues from Endophyte-Infected Ryegrass

available evidence is consistent with the metabolic grid depicted in Figure 4. It is conceivable that the lolicines could be metabolized to lolitriol (4a), and then to the lolitrems (e.g., 3a), by the endophyte (Figure 4). This is an important observation; biosynthesis of the lolitrems may proceed without the intermediacy of 7 or 8, and proposed screening tests based on the inability of endophytes to produce paxilline analogues (Garthwaite et al., 1993; Penn et al., 1993; Gurney et al., 1994) might therefore fail to detect endophytes capable of producing lolitrems or other tremorgenic indole-diterpenoids. It is interesting, therefore, to note that although A. yamanashiensis produced an array of terpendoles (close analogues of the lolitrems), of which at least one was tremorgenic (Munday-Finch et al., 1997), as well as the presumed lolitrem precursors paspaline (5a) and emindole SB, neither paxilline (7) nor R-paxitriol (8) was identified among its metabolites (Huang et al., 1995a; Tomoda et al., 1995). Given that 5a appears to be the crucial intermediate in the biosynthesis of all indole-diterpenoid tremorgens (Mantle and Weedon, 1994; Munday-Finch et al., 1996a), it might be advantageous for plant breeders to select endophytes on the basis of their ability to produce paspaline analogues; endophytes incapable of biosynthesizing paspalines would be incapable of producing any indole-diterpenoid tremorgen, regardless of their host grass. Several grasses infected with Acremoniumlike endophytes cause staggers syndromes similar to RGS, but contain only unidentified non-lolitrem indolediterpenoids (Fletcher et al., 1993; Miles and di Menna, 1995; Miles et al., 1995a,e), indicating that endophyte toxins other than lolitrems are capable of causing staggers. A biosynthetically based screening strategy, involving analysis for paspaline analogues (e.g., by ELISA), would detect endophytes capable of producing such toxins and thereby facilitate the endophyte selection process. ACKNOWLEDGMENT

We thank J. M. Allen, B. Clarke, and D. E. McNaughton for obtaining mass spectral data. Supporting Information Available: 1H NMR spectra and HMBC correlations of lolicine A 11-O-propionate (1b), lolicine B 11-O-propionate (2b), and lolitrem N 10-O-propionate (4d); 1H and COSY NMR spectra of lolitriol 10-O-acetate (4b); HMBC correlations and ROESY NMR spectrum for 3a; and calculated H-17-H-36 interatomic distances for 3a and 3c (14 pages). Ordering information is given on any current masthead page. LITERATURE CITED Braun, S.; Kalinowski, H.-O.; Burger, S. 100 and More Basic NMR Experiments; VCH Verlagsgesellschaft mbH: Weinheim, Germany, 1996; pp 344-392. Breitmaier, E.; Voelter, W. Carbon-13 NMR Spectroscopy. High-resolution methods and applications in organic chemistry and biochemistry, 3rd ed.; VCH Verlagsgesellschaft mbH: Weinheim, Germany, 1987; p 315. Cleland, J. B. Plants poisonous to stock in Australia. In Third Report of the Bureau of Microbiology; New South Wales Government: New South Wales, Australia, 1912; pp 187217. di Menna, M. E.; Sprosen, J. M.; Hawkes, A. D.; Miles, C. O. Endophyte culture production of lolitrem B and derivatives. In Mycotoxic Diseases Group Annual Report 1990/91;

J. Agric. Food Chem., Vol. 46, No. 2, 1998 597 Ruakura Agricultural Research Centre, Ministry of Agriculture and Fisheries: Hamilton, New Zealand, 1991; pp 9-10. Ede, R. M.; Miles, C. O.; Meagher, L. P.; Munday, S. C.; Wilkins, A. L. Relative stereochemistry of the A/B rings of the tremorgenic mycotoxin lolitrem B. J. Agric. Food Chem. 1994, 42, 231-233. Fletcher, L. R. Grazing ryegrass/endophyte associations and their effect on animal health and performance. In Proceedings of the Second International Symposium on Acremonium/Grass Interactions: Plenary Papers; Hume, D. E., Latch, G. C. M., Easton, H. S., Eds.; AgResearch: Palmerston North, New Zealand, 1993; pp 115-120. Fletcher, L. R.; Harvey, I. C. An association of a Lolium endophyte with ryegrass staggers. N. Z. Vet. J. 1981, 29, 185-186. Fletcher, L. R.; Garthwaite, I.; Towers, N. R. Ryegrass staggers in the absence of lolitrem B. In Proceedings of the Second International Symposium on Acremonium/Grass Interactions; Hume, D. E., Latch, G. C. M., Easton, H. S., Eds.; AgResearch: Palmerston North, New Zealand, 1993; pp 119-121. Freeman, E. M. The seed fungus of Lolium temulentum, L., the darnel. Proc. R. Soc. London 1902, 71, 27-30. Gallagher, R. T.; White, E. P.; Mortimer, P. H. Ryegrass staggers: isolation of potent neurotoxins lolitrem A and lolitrem B from staggers-producing pastures. N. Z. Vet. J. 1981, 29, 189-190. Gallagher, R. T.; Campbell, A. G.; Hawkes, A. D.; Holland, P. T.; McGaveston, D. A.; Pansier, E. A.; Harvey, I. C. Ryegrass staggers: the presence of lolitrem neurotoxins in perennial ryegrass seed. N. Z. Vet. J. 1982, 30, 183-184. Gallagher, R. T.; Hawkes, A. D.; Steyn, P. S.; Vleggaar, R. Tremorgenic neurotoxins from perennial ryegrass causing ryegrass staggers disorder of livestock: structure elucidation of lolitrem B. J. Chem. Soc., Chem. Commun. 1984, 614616. Gallagher, R. T.; Hawkes, A. D.; Stewart, J. M. Rapid determination of the neurotoxin lolitrem B in perennial ryegrass by high-performance liquid chromatography with fluorescence detection. J. Chromatogr. 1985, 321, 217-226. Garthwaite, I.; Miles, C. O.; Towers, N. R. Immunological detection of the indole diterpenoid tremorgenic mycotoxins. In Proceedings of the Second International Symposium on Acremonium/Grass Interactions; Hume, D. E., Latch, G. C. M., Easton, H. S., Eds.; AgResearch: Palmerston North, New Zealand, 1993; pp 77-80. Gatenby, W. A.; Munday-Finch, S. C.; Wilkins, A. L.; Miles, C. O. Terpendole M, a novel indole-diterpenoid isolated from Lolium perenne infected with the endophytic fungus Neotyphodium lolii. J. Agric. Food Chem. 1997 (in preparation). Gurney, K. A.; Mantle, P. G.; Penn, J.; Garthwaite, I.; Towers, N. R. Loss of toxic metabolites from Acremonium lolii, the endophyte of ryegrass, following mutagenesis. Naturwissenschaften 1994, 81, 362-365. Henry, M.; Massey, A. E. Some neglected sheep diseases of New South Wales. Agric. Gaz. N. S. W. 1911, 22, 109-117. Hoveland, C. S.; Haaland, R. L.; King, C. C., Jr.; Anthony, W. B.; Clark, E. M.; McGuire, J. A.; Smith, L. A.; Grimes, H. W.; Holliman, J. L. Association of Epichloe typhina fungus and steer performance on tall fescue pasture. Agron. J. 1980, 72, 1064-1065. Huang, X.-H.; Nishida, H.; Tomoda, H.; Tabata, N.; Shiomi, K.; Yang, D.-J.; Takayanagi, H.; O h mura, S. Terpendoles, novel ACAT inhibitors produced by Albophoma yamanashiensis II. Structure elucidation of terpendoles A, B, C and D. J. Antibiot. 1995a, 48, 5-11. Huang, X.-H.; Tomoda, H.; Nishida, H.; Masuma, R.; O h mura, S. Terpendoles, novel ACAT inhibitors produced by Albophoma yamanashiensis I. Production, isolation and biological properties. J. Antibiot. 1995b, 48, 1-4. Mantle, P. G.; Weedon, C. M. Biosynthesis and transformation of tremorgenic indole-diterpenoids by Penicillium paxilli and Acremonium lolii. Phytochemistry 1994, 36, 1209-1217.

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Received for review August 8, 1997. Revised manuscript received November 20, 1997. Accepted November 25, 1997. JF9706787